1. Field of the Invention
This invention relates to a liquid crystal projection display system comprising at least one liquid crystal display panel for modulating light to produce a display output, means for directing light onto the display panel, a display screen, and a projection lens for projecting the display output from the display panel onto the display screen.
2. Discussion of the Related Art
Liquid crystal projection display systems are well known. A single monochrome or multi-colour display panel may be used. Alternatively, three separate display panels may be used, each illuminated with light a respective colour, usually red, green and blue, whose outputs are combined at the display screen to produce a full colour display. The light directed onto the display panel is modulated according to the transmission characteristics of individually-driven picture elements in an array which together produce the required display, for example a datagraphic display or a TV picture. The display panel may be operated in a transmissive mode, in which illuminating light passing through the panel from one side to the other is modulated, or in a reflective mode, in which illuminating light is directed onto one side and the display output is obtained from the same side.
One problem with liquid crystal display panels is that many do not give their best contrast when viewed at normal incidence, i.e. when viewed along a direction perpendicular to the plane of the panel. Better contrast is obtained when viewing a panel at around, for example, 5 to 10 degrees away from the normal (at a predetermined azimuth). Most known liquid crystal projection display systems are arranged such that the object, that is, the display panel, is perpendicular to the projection lens axis, assuming the light output from the panel is supplied directly to the projection lens without an intervening mirror or the like being employed. In some systems a mirror may be employed so as to fold the light path, for example to produce a more compact system. Even so, the direction of view is effectively the same.
It has been suggested previously that the liquid crystal panel can be inclined, that is, tilted, with respect to the main optical axis of the projection lens so as to obtain a higher contrast output. The tilting angle would typically be around 5.degree. to 10.degree.. In the English language abstract of Japanese Kokai 3-75617 there is described a transmissive mode projection system of the kind using three separate display panels and mirrors in which light from a common source is split into three different colour components, each of which is directed along a respective optical path between the common light source and a common projection lens onto a display panel and in which each panel is inclined with respect to the optical central axis of the optical path associated with that panel with a condenser lens being placed in each optical path immediately before the display panel. The tilting of the display panels in this way, however, introduces focusing and distortion problems in that the projected display image, although having higher contrast, will be largely out of focus, except, for example, along the line of the tilt axis, and will also suffer from keystone distortion.
In the English-language abstracts of Japanese Kokai's 3-259131 and 4-80710 there are described transmissive mode projection display systems in which light from a source is directed through a liquid crystal display panel at a small angle with respect to the normal. An optical wedge is placed adjacent the light output side of the panel to deflect the output beam from the panel such that its direction then coincides substantially with the main optical axis of the projection lens and so that the image projected on the projection screen suffers less distortion. However, in these systems the planes of the display panel and the projection screen are apparently substantially parallel to one another and consequently it can be expected that the displayed image will be largely out of focus. In the projection system described in the abstract of Kokai 4-80710, a further optical wedge is positioned adjacent the light input side of the liquid crystal panel which serves to deflect a beam of light from a light source whose optical axis is substantially parallel to the central axis of the projection lens such that the illuminating beam is incident on the liquid crystal panel at the required small angle with respect to the normal for contrast enhancement, the deflection by this further wedge being in the opposite sense to the deflection produced by the optical wedge adjacent the output side of the panel. In the English language abstract of Japanese Kokai 3-43780 there is described a transmissive mode projection system in which light from a source passes through an optical wedge before passing through a liquid crystal panel and then through another optical wedge after the liquid crystal panel. The wedge angles are in the plane that contains the normal to the liquid crystal panel and the optimum visual field angle direction, and so arranged that light passes through the liquid crystal panel at angles equally distributed on either side of the optimum visual field angle. As the planes of the liquid crystal panel and the viewing screen are apparently substantially parallel to one another, and an optical wedge is between the panel and the projection lens, it can be expected that the displayed image will be largely out of focus and distorted.
While in these known systems it might be possible to tilt the image, i.e. the screen, to bring the image back into focus, this would result in distortion of the displayed image.
Another transmissive mode liquid crystal projection system is described in the English-language abstract of Japanese Kokai 63-73782 in which an optical wedge is positioned adjacent the input side of a liquid crystal panel to deflect a beam of light from a light source such that it is incident on the panel at a small angle to the normal. A projection lens has its optical axis arranged parallel to the light beam before deflection and projects the display light output onto a screen which is disposed parallel to the panel. In this system, the centers of the screen and the panel and the central axis of the projection lens are all being shifted with respect to one another and to the central axis of the light beam from the light source. While the extent of keystone distortion may be reduced by such shifting, the system suffers from the drawback that it demands a projection lens with a significantly larger field of view than normal which would be of complex and difficult design and consequently expensive.